1. Technical Field
This technology pertains generally to spin-based nano-electronic devices and systems that use the orbital properties of electrons rather than their charge, and more particularly to laminates producing spin-orbit torques that can switch the magnetization in perpendicularly magnetized laminates without the need for an external magnetic field.
2. Background Discussion
In the emerging technology of spintronics, short for “spin electronics,” not only are electron charges utilized, but the intrinsic spin of the electrons and the associated magnetic moments are utilized as well. The aims of spintronic applications are to control, manipulate and measure the magnetization of nanostructures using the spin of an electric current.
In principle, manipulating spin or magnetization should require far less energy than is required to move charge, should be faster and can take place at very small scales. Thus, the utilization of the electron's spins has advantages over the conventional electronic devices such as low power consumption, and non-volatility.
Spin transfer torque (STT) and spin-orbit torque (SOT) driven magnetization dynamics provide two different approaches to the creation and manipulation of nanoscale magnetic devices. Spintronic devices generally employ a magnetic material in multilayers to utilize the spin of the charges. Current flowing through one layer is spin polarized and the polarized current interacts with the magnetization of adjacent layers.
In the case of the spin transfer torque effect, angular momentum from electrons in a spin polarized current is transferred to the localized d-electrons that hold the magnetization in a ferromagnetic film. The electrons exert a net effective torque on the magnetic moment of the ferromagnetic material, generally called spin transfer torque (STT), as a result of the conservation of angular momentum. However, the level of current density needed to reorient the magnetization using STT is presently too high for most commercial applications or contributes to a reduction in the useful life of the devices. Conventional STT-MRAM devices, for example, experience rapid aging of the tunnel barriers produced by high writing current densities.
In contrast to the transference of spin angular momentum between two non-collinear magnetic layers or domains with STT, the spin-orbit torque (SOT) effect involves the transfer of orbital momentum from the lattice to the spin system.
Current-induced magnetic manipulation and switching is possible with the injection of an in-plane current into the non-magnetic layer that produces a perpendicular (out of plane) spin current which is transferred to the magnetization in an adjacent ferromagnetic layer creating a spin torque. Spin current generation may arise from the metal layer by the spin Hall effect or it may arise at the interface by current induced spin polarization (the Rashba-Edelstein effect).
Spin orbit torques (SOTs) are also of interest because they can lead to magnetization switching in geometries that are not possible with conventional spin transfer torque (STT) devices.
Magnetization switching by current-induced spin-orbit torques is of great interest due to its potential applications in ultralow-power memory and logic devices. These devices need the SOT effects to switch ferromagnets with a perpendicular (out-of-plane) magnetization. At the present time, however, the presence of an in-plane external magnetic field is typically required to assist SOT-driven switching and this is a major obstacle for any practical application. In conventional devices, the external field allows each current direction to favor a particular orientation for the out-of-plane component of magnetization, thereby resulting in deterministic perpendicular switching. However, this external field is undesirable from a practical point of view because it also reduces the thermal stability of the perpendicular magnet by lowering the zero-current energy barrier between the stable perpendicular states, resulting in a shorter retention time if used for memory, for example. A critical requirement to achieve high-density SOT memory, therefore, is the ability to perform SOT-induced switching without the use of external magnetic fields, in particular for perpendicularly-magnetized ferromagnets, which show better scalability and thermal stability as compared to the in-plane case. However, there are currently no practical solutions that meet this requirement.
Accordingly, there is a need for devices that produce SOTs that can switch the magnetization in perpendicularly magnetized films without the need for an external magnetic field. The technology described herein satisfies this need as well as others and is generally an improvement in the art.